A glass cabling media that sends network signals using light. Fiber-optic cabling has higher bandwidth capacity than copper cabling, and is used mainly for high-speed network Asynchronous Transfer Mode (ATM) or Fiber Distributed Data Interface (FDDI) backbones, long cable runs, and connections to high-performance workstations.
Fiber-optic cabling consists of a signal-carrying glass core of 5 to 100 microns in diameter (a sheet of paper is about 25 microns thick and a human hair about 75 microns thick), surrounded by a layer of pure silica called cladding, which prevents light from escaping.
Surrounding the cladding are protective layers of acrylic plastic coating, Kevlar fibers for additional strength, and a PVC (polyvinyl chloride) jacket (usually colored a distinctive orange).
Network components use LED or laser diodes to convert electrical signals into light pulses for transmission on fiber-optic cables. An optical detector is used to convert the light pulses back into electrical signals.
Graphic F-5. Fiber-optic cabling.
There are two types of fiber-optic cabling:
Connectors for fiber-optic cabling come in several varieties, including SC, ST, and SMA connectors. ST connectors have a wider installed base, but SC connectors are more versatile and are becoming more popular. SMA connectors do not conform to EIA/TIA wiring standards.
Fiber-optic cabling has several advantages over copper cabling, including the following:
Graphic F-6. Connecting two LANs using fiber-optic cabling.
Fiber-optic cabling is often used for campus-wide backbones, long cabling runs between buildings, and local area network (LAN) connections to heavily used servers or high-speed workstations. Fiber is used also in heavy industrial environments where machinery can cause high levels of EMI. Fiber is not used extensively at the LAN level yet because it is more expensive and more difficult to install than copper cabling. Long-distance telecommunications carriers such as Sprint and MCI use fiber-optic cabling exclusively for their country-wide telecommunications lines.
Different styles of fiber-optic cabling exist, depending on the intended use. Examples include the following:
Line drivers for fiber-optic cabling are available for synchronous or asynchronous transmission as well as for single-mode or multimode fiber, allowing you to extend or interconnect LANs in either point-to-point or multipoint configurations.
Remember that the bandwidth of a fiber-optic cable depends on the distance as well as the frequency. Bandwidth is usually expressed in frequency distance form, for example in MHz-km. In other words, a 500-MHz-km fiber-optic cable can transmit a signal a distance of 5 kilometers at a frequency of 100 MHz (5 x 100 = 500), or a distance of 50 kilometers at a frequency of 10 MHz (50 x 10 = 500). In other words, there is an inverse relationship between frequency and distance for transmission over fiber-optic cables.
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Be careful not to unduly stress fiber-optic cabling during installation. The maximum acceptable bend radius is usually 20 times the diameter of the cable. Use an optical time domain reflectometer (OTDR) to test for faults after installation. Loss of signal, or attenuation, in fiber-optic cables can be caused by absorption (no medium is completely transparent to light), cable microbending (especially in single-mode fiber if it is not installed correctly), connector loss because of poor splicing or poorly installed or misaligned connectors, or coupling loss at the transmitter or receiver.
For safety, never look down a fiber-optic cable connected to your network because the invisible laser light can injure the retina of your eye. When splicing connectors onto fiber, be careful to avoid getting shards of glass in your eyes or on your hands - use double-sided tape to clean the connection and remove loose shards. Wear protective eyewear.
Fiber-optic cabling is available for purchase in bulk for those who want the challenge of terminating it themselves, but most customers buy standard or custom preterminated cables from suppliers. These cables can be simplex or duplex; they can be single-mode or multimode (multimode is most common); and they can be terminated with ST-ST, ST-SC, SC-SC, or SMA connectors.